Oh man there's so many I just get irritated seeing. Tom Cruise is on my list of people I really don't want to see in anything, ever. Orlando Bloom I don't see as being as bad as his current reputation suggests, Legolas remains a favourite from the LOTR movies even with him playing the part, and he wasn't bad in POTC either.
The trouble for me is I rarely remember names, so I can't provide many examples of the ones I dislike..
Jack Whitehall, a british comedian, has recently started acting in some Channel 4 shows, but he's only mainstream in a British sense as far as I'm aware so he probably doesn't count..
Matt Damon, I would put up there as well.
And, while I may get some serious hate for this, Simon Pegg. Hollywood has gone to his head, and to be quite frank he is shite without Nick Frost supporting him.
Although worst is a bad choice of word, since everyone's opinion differs, so I went with "most damn annoying to see given a role in a film, ever."